It is known that the properties of semiconductor devices, such as a touchscreen display, a photo sensor, an integrated optical (photonic) circuit, a light emitting diode (LED), etc. are improved by applying a transparent thin film having a high refractive index to the light-emitting part, the light-sensitive part, or the like of the device structure. Known materials for such a transparent thin film having a high refractive index include organic polymers, organic-inorganic hybrid polymers, inorganic metal oxides, etc.
Organic polymers can easily be formed into a thin film at a low temperature, and provide excellent mechanical properties including high surface adhesiveness. The refractive indices of organic polymers in the visible wavelength range are as high as about 1.65 to 1.70, but this level is close to the upper limit. In addition, these high-refractive-index polymers have limited thermal and chemical stabilities because they generally contain a high concentration of highly-polar atoms, such as bromine, iodine, or sulfur atoms.
An organic-inorganic hybrid polymer is obtainable by dispersing nano-sized (1 to 50 nm in diameter) metal oxide particles in a carrier polymer, and has an increased refractive index without the loss of original transparency of the polymer. However, it is difficult to achieve a refractive index comparable to that of the dispersed metal oxide alone, and the refractive index of an organic-inorganic hybrid polymer in the visible wavelength range is about 1.70 to 1.80, which is close to the upper limit.
Among transparent metal oxides having high refractive indices, titanium oxide and zirconium oxide are best known. When used as thin films, these metal oxides exhibit particularly high optical transparency, and the refractive indices thereof are 2.0 or more in the visible wavelength range. However, in order to form thin films of these metal oxides, deposition by an expensive and inefficient method, such as vaporization or sputtering, is required in general. Furthermore, a deposited metal oxide film is brittle and does not tend to firmly adhere to the device surface.
JP 2007-521355 T discloses a method for producing a thin film of a metal oxide by applying, onto a substrate, an organic polymer solution in which an organometallic oligomer composing a chelate complex is dispersed, heating the coating for the curing thereof, and further heating the cured film for the decomposition of organic matter. However, this method is not suitable for mass production for the reason that the thermal curing of the coating hampers quick and accurate patterning.